Fuel economy gone down

[00pathyse]

Hello,

I've noticed the fuel economy in my 2007 Altima 3.5 has steadily gone down over the last few years. 21 mpg at best on the highway, 15mpg city on premium fuel. No driveability issues whatsoever and no engine lights. I've checked the brakes and wheel bearings and tire pressure and all are good. 75000 miles on the clock. Any other obvious things I should look into? Any suggestions appreciated. Thanks.

[VStar650CL]

Have you ever given it an IAVL to recalibrate the MAF? Modern MAF's pretty much all drift electrically as the electronics age, sometimes by a lot. In a Nissan where the ECM has no MAP and is totally dependent on an accurate MAF, bad things can happen to both mileage and performance without a periodic recalibration.

[00pathyse]

I was thinking about that. I have the cvtz50 app, but that function has never worked for me for some reason. Maybe it's not supported for 07? Ive also tried the manual procedure that's in the fsm with no luck. At least I don't think it did anything. I'll give it another try...

[VStar650CL]

You still need to satisfy the conditions for an IAVL to take:
Engine temp > 70C
Throttle released, engine RPM < 1000
CVT in Park, fluid temp > 40C
All loads off (blower, headlamps, A/C)

If you're missing any of those, it will sit there in "duh" mode and eventually time out.

[00pathyse]

Thanks , I was in duh mode. I cleaned the throttle body and did the idle vol relearn (after warm up) on a free app called nds3 lite. Worked like a charm. I scanned for codes and got a pending code p0113. Was that related to the hard start I had right after cleaning the throttle body due to the remaining cleaner left in there? There is no check engine light on.

[VStar650CL]

P0113 implies that your MAF grounds are going bad. The IAT is built into the MAF and they share grounds at the ECM through a joint connector. Do a voltage drop test on both of the Green/Black wires at the MAF connector and see if one or both read high. The MAF wires have a habit of breaking on older rides from constant flexing whenever the air filter gets changed or the airbox is removed. If you find crappy grounding, there's nothing wrong with t-tapping a supplemental ground to the block or chassis.

[VStar650CL]

PS - The IAT reading high (cold) will definitely trash your fuel mileage.

[00pathyse]

Are you talking about disconnecting the connector and checking from inside the plug? I get 0v on both black/green wires with car on but not running.

[VStar650CL]

No, voltage drop needs to be done by back-probing the connector with the engine running. Since it's a ground circuit and won't draw a lot of current, you can also ohm it to ground with the connector loose and the engine off. The reading should be below 1 ohm, if it's higher then there's a ground issue. But before you do that, do a voltage drop test on your block and chassis to make sure you don't have a generic ground issue. With the engine running, put your meter on a low scale and measure from the negative post to the block and chassis. The readings must be 50 millivolts (0.05V) or lower. If they're higher in either spot, your ground cabling has a problem.

[00pathyse]

Thanks for all your help.

I have .007v from post to engine block and 0v from post to frame.

Both black/green wire have .96 ohms. What was the third wire to check?

[00pathyse]

Sorry, that should be .096 ohms on the smallest scale on my meter

[VStar650CL]

No third wire, there are only the two grounds. That sounds healthy. Will the code erase? Did you maybe cycle the key with the MAF disconnected while you were cleaning the TB? If so then it's weird that there wouldn't be a P0102/P0103 with it, but Hitachi doesn't exactly give us flowcharts for their code logic. I wouldn't bet my house on it being impossible.

[00pathyse]

Yea, I probably did cycle it while cleaning. Darn, thought I was getting somewhere with the fuel economy. Well, at least I can rule out the maf, and tb. Thanks once again Vstar!

[VStar650CL]

Now that you did the IAVL, see if it improves. Case is probably still out on that. Might pay you to check the IAT resistance, too. That should be about 2 Kohms +/- 10% at room temperature (25C). An IAT that reads high (lower resistance than normal) will usually just cause cold stumble, but one that reads low (higher resistance than normal) will have a very marked effect on fuel economy. They do sometimes fail, that was one of the first things I had to replace on the wife's Altie when we first bought it at 65K miles.

[00pathyse]

I measured the IAT resistance and it was right around 1800 ohms it is about 19C in here right now. Would that mean it is border line low? I blew in it to warm it up and went to about 1600. Also, I noticed my cold start idle is a little higher than it was before the tb clean. It jumps to about 2100rpm and then starts to settle down. Warm idle is good. Is there a procedure to bring the cold start idle down or is that good? Thanks.

[VStar650CL]

Yes, 1.8K at 19C is right on the edge of the graph, if it isn't a bad sensor then it's very close. The cold idle speed isn't really a worry as long as it settles down quickly.

[00pathyse]

Since its reading low, not high, would that just cause the cold stumble? Not the poor fuel economy?

[VStar650CL]

Correct.

[00pathyse]

Any suggestions for next things to look at in regards to fuel consumption?

[VStar650CL]

Has it ever been decarboned? Carbon buildup on the crowns will increase the compression ratio and make the ECM slightly retard the timing and slightly rich the mixture in response to knocking that your ears can't hear.

[00pathyse]

No, what does that entail? It does run premium almost exclusively.

[VStar650CL]

With carbon it's more a matter of how hard it's driven. Decarboning is easy, I put a teaspoon of ATF down the brake booster hose while an assistant holds the RPM's at about 4K with the engine warm. The mineral oil vapor coats the brittle carbon and creates a temperature differential that breaks it apart.

[00pathyse]

Maybe a stupid question, but is the ATF bad for the rubber or anything else? Maxlife ATF ok?

[VStar650CL]

Any kind, even CVT fluid. They're all just mineral oil under the skin. You only need a little, and the fluid and the dislodged carbon will burn up in the cats. The only risk is if the carbon on the valves is very heavy, sometimes a chunk will get trapped between the piston crown and head and squash the spark plug electrode, but that happens very rarely on modern port-injected engines. These days it's mostly a worry with DGI engines, which carbon-up much faster than MPI, especially if they're driven like grandma.

[00pathyse]

I'm finally going to get to doing this. How long should I rev it to 4k rpm? I had a look at the plugs and they do look a little carboned, and I can see some carbon buildup through the hole.

[VStar650CL]

You don't need to hold it long, just make sure the engine is good and warm before you start the procedure. In a fully warm state, breakup of the carbon will be almost instantaneous.

[00pathyse]

I got to this today, no issues thanks. Hope it helps!

[VStar650CL]

[00pathyse]

My spark plugs are pretty covered in black soot, more than I've seen on any other car I've owned. This car does frequent short trips so this could have something to do with it. But I want to checkout the a/f sensors. I have a obdlink MX+, what should I be looking at for proper operation of the af sensors? There is a few different parameter options on my obdlink app and nds3 app, is there a certain parameter I need to look at or is it just voltage? Thanks.

[VStar650CL]

The A/F's respond directly to mixture, they don't oscillate like HO2S sensors do. Nissan uses two different biases, earlier ones are 1.5V and later ones are 2.2V. I'm not sure which one your '07 uses, but if you look up an A/F code in the manual there will be a chart. They both work identically, the bias level represents stoichiometry and above it means lean, below it means rich. The usual failure mode is that they get slow to respond, which can be difficult to diagnose on an L4 but will usually show up by comparing the banks on a V6 or V8. Problems with them can also be caused by pinholes in the exhaust near the sensor, so before condemning one, have an assistant block the exhaust with a shop rag and see if there are any leaks nearby.